There is an ongoing need to improve the electrical characteristics of capacitors. Two long term trends in the electronics industry are the on-going miniaturization and reduced cost of the components. For solid electrolytic capacitors increased volumetric efficiency and reduced cost are achieved primarily through the use of higher specific surface area powders of the valve metals used to form the anode body. As the specific surface area of the anode body increases, the pore diameters decrease which presents challenges in the manufacturing process.
The anode of a typical solid electrolytic capacitor consists of a porous anode body, with a lead extending beyond the anode body and connected to the positive mounting termination of the capacitor. The anode is formed by first pressing a valve metal powder into a pellet. Valve metals include Al, Ta, Nb, Ti, Zr, Hf, W, and mixtures, alloys or suboxides of these metals. The anode is sintered to form fused connections between the individual powder particles.
The dielectric is formed on the internal and external surfaces of the porous anode through an anodic oxidation process by application of voltage to the anode while immersed in an electrolyte solution. The thickness of the dielectric film is proportional to the applied voltage.
The cathode of a solid electrolytic capacitor is typically either manganese dioxide or an intrinsically conductive polymer. In either case the internal surfaces of the dielectric are coated with the cathode material by first dipping the anode body in a solution(s) that is subsequently converted to the solid cathode. The dipping process is termed impregnation. In the case of manganese dioxide cathode the anodes are dipped in manganese nitrate solutions which are subsequently converted to manganese dioxide in a thermal decomposition process. This process is commonly called conversion. Intrinsically conductive polymers are formed on the internal surfaces of the dielectric by dipping the anodes in a solution of the monomer and a solution of an oxidizer, either in a single co-mixed solution or as separate solutions in an alternate dip process. Once the monomer and oxidizer impregnate the anode body the polymerization reaction is allowed to occur so that the intrinsically conductive polymer coats the internal dielectric, surfaces. The process by which the oxidizer and monomer are allowed to react is commonly referred, to as polymerization.
The capacitance of a solid electrolytic capacitor is governed by the general equation of capacitance:C=kA/d where C=capacitancek=dielectric constantA=surface area of the anode/cathode plates,d=distance between the anode and cathode plates or dielectric thickness.
Since the distance between the anode and cathode plate is proportional to the voltage used to form the dielectric in the anodic oxidation process, the formation voltage (Vf) can be substituted for d in the general equation for resistance. Furthermore, since the dielectric constant is a material property of the dielectric it can be seen that the multiplicative product of capacitance and formation voltage is proportional to the surface area of the anode. This product is commonly referred to in the industry as CV. The specific surface area of commercial valve metal powders used to manufacture solid electrolytic capacitors is expressed as the product of capacitance times formation voltage divided by the powder weight. This measure of specific surface area of a valve metal powder is commonly referred to as the charge of the powder and it is often abbreviated as CV/g.
In order to drive the on-going need for miniaturization of electronic components and assemblies, valve metal manufacturers have developed ever higher CV/g powders over the last 40 years as indicated in FIG. 1. As the CV/g has increased, the diameter of the pores has decreased. Coating the internal dielectric surfaces with the solid cathode is very difficult for the high CV/g powders, available, today due to the decreased pore size. Incomplete coating of the internal dielectric surface with the solid cathode results in a loss of capacitance in the finished device. The loss of capacitance due to the incomplete cathode coverage is expressed as capacitance recovery and is defined by Equation 1.Capacitance recovery=100×(dry capacitance/wet capacitance)  Equation 1
Wet capacitance is determined by the amount of Ta in the anode, the specific surface area of the Ta (CV/g) and the anodization voltage. The equation for calculating wet capacitance from these anode characteristics is calculated in accordance with equation 2.Wet capacitance=(Ta volume×press density×CV/g)/(formation voltage)  Equation 2
Dry capacitance is the capacitance measured after application of the solid electrolyte when the anode is in a dry state.
The buildup and uniformity of the solid electrolyte inside the pores of the anode impacts several of the electrical characteristics of the finished device which effect the performance of the capacitor in an electric circuit. Poor or nonuniform buildup causes the resistance of solid electrolyte inside the pores to increase, resulting in increases in equivalent series resistance (ESR) and dissipation factor (DF), and a loss in capacitance at higher frequencies (capacitance roll off). DF is the parameter most often used to measure this characteristic of the solid electrolyte.
Another means of reducing the size of the anode is to increase the anode density. Thus for a given CV/g powder the CV/cc is higher as the anode density is increased. However, as the anode density increases the pore diameters decrease and complete coating of the dielectric surfaces is difficult.
The ability to coat the internal dielectric surfaces is also governed by the size of the anode. For example 150,000 CV/g powders are currently used in commercially available capacitors with anode bodies less than 0.015 cubic centimeters wherein the smallest dimension is approximately 1 mm. However, for anodes greater than approximately 0.05 cubic centimeters, where the smallest dimension is approximately 3.3 mm, the practical limit for CV/g powders in use today is 70,000 CV/g.
Manufacturers of solid electrolytic capacitors have been improving the processes for coating the dielectric surfaces for years. Factors manipulated to improve the impregnation process include solution concentration, dip times, dip speeds, surface tension, and vacuum impregnation. Despite these improvements capacitance recover is less than 50% for large case size anodes pressed from powders with CV/g exceeding 60,000. For capacitors employing manganese dioxide as the cathode, this is largely due to the redistribution of the manganese dioxide during the conversion process. This redistribution occurs in part due to the evolution of gasses which occurs during the reaction:Mn(NO3)2→MnO2+2NO2(g)
As these gases escape from the internal body of the anode they carry unreacted manganese nitrate to the outer portions of the anode body. This results in poor coverage of the dielectric surfaces close to the center of the anode and poor capacitance recovery. Similarly as the solvents in the monomer and/or oxidizer solution(s) evaporate prior to, or during, polymerization the evolved gases cause a redistribution of the polymer away from the center of the anode body.
Another important trend in the capacitor industry is the drive to components with low ESR at high frequencies (100 k Hz and higher). In order to reduce ESR, component manufacturers develop means of reducing the resistance of the various elements of the capacitor. Typically the largest contribution to the ESR of the component is due to the resistance of the internal and external cathode layers. The resistance of these elements follow the general equation of resistance as reproduced in Equation 3.Resistance=resistivity×path length/cross sectional area  Equation 3
Fluted anodes comprising a furrow or groove on the otherwise monolithic capacitor body as described, for example, in U.S. Pat. Nos. 6,191,936, 5,949,639 and 3,345,545 reduce the path length through the internal cathode layer and increase the cross-sectional area for current to flow through the external cathode layer. Capacitors utilizing fluted anodes as illustrated in FIG. 1 enjoy much success and this technique is still utilized in current capacitors. However, the groove cut in the anode reduces the CV of the anode, resulting in lower capacitance of the device.
There has been an ongoing desire to improve impregnation of the anode body thereby allowing for improved capacitance and the ability to take full advantage of higher CV/g powders without sacrifices in equivalent series resistance, dissipation factor or capacitance roll off.